Have you heard of freshwater jellyfish?

Have you heard of freshwater jellyfish?

By By Dr. Robert A. Hedeen

By Dr. Robert A. Hedeen


Almost everyone who has ever been subjected to a course in biology, in either high school or college, will recall that jellyfish and their relatives are strictly creatures of the sea. An exception exists: the almost microscopic, freshwater, polyp-like Hydra, which beginning biology students routinely subject to intensive study and experimentation.

But, few, including professional biologists, are aware of another, somewhat larger exception: a species of jellyfish found only in fresh water. If you have not had the opportunity to become acquainted with one of these creatures, don’t feel badly as they are quite rare, and anyone running into one in nature is privy to a singular experience.

The “jaw-busting,” scientific name of the freshwater jellyfish is Craspedacusta sowbyi, and it was first found in England in 1880; more recently it has been noted in several other locations in Europe. The Craspedacusta sowbyi was first discovered in this country in 1908, and since that time it has been found in about 60 localities, generally east of the Mississippi River. This jellyfish has been reported from a variety of fresh-water environments including farm ponds, small lakes, and water-filled areas in stone quarries.

I have been fortunate in having encountered this rare animal on two separate occasions; once in 1968 in a pool in a limestone quarry in southwestern Cook County and again in 2001 in a sandstone-based pool in a rocky gorge at Franklin Creek Natural Area in Ogle County.

This primitive creature, which may vary in size from about 1/4 of an inch to almost an inch in diameter, is an enigma to biologists who study it. Like its numerous salt-water relatives, the jellyfish, or medusa as it is properly called, represents the sexually reproducing stage of an animal that has two different forms and life styles.

The free-swimming medusa arises from a tiny polyp-like form that lives attached to some solid object on the bottom of the impoundment. Because of its extremely small size, the polyp stage has been viewed in nature only a few times.

Normally, the attached polyp creates buds, without the benefit of sex, which change into small, free-swimming jellyfish which are either male or female. As the jellyfish mature, sex cells (sperm and eggs) are produced and are shed into the water. The fertilized egg grows into a tiny larva which, after swimming around for a while, settles down on a fixed object and develops into the polyp stage. In similar marine animals, this reproductive cycle is on a regular, alternating basis, with one type of reproduction giving rise to the other in the next generation (asexual reproduction alternating with sexual reproduction).

In fresh-water jellyfish, however, this is apparently not the case, as the appearance of the medusa or jellyfish stage is highly sporadic and unpredictable. In some years, jellyfish may be found in abundance in certain ponds or pools, but in subsequent years they may be absent, only to reappear several years later or in several successive years. In other locations, they have been found only once and have never reappeared in spite of careful searching. Or, a pond may be found to be teeming with these little blobs of protoplasm, and another similar pond only a few yards away will be devoid of them.

Fresh-water jellyfish possess the characteristic stinging cells of their marine relatives, but the venom produced has no affect on humans. Small prey animals such as water fleas, however, are not as lucky as they are quickly immobilized by the toxin when they encounter a stinging cell.

Some 450 million years ago, at the end of the Silurian Period in geological history, the sea that covered this part of the United States began to recede. In some unknown way, today’s freshwater jellyfish managed to part company with the salt-water environment in which they evolved and became adapted to a freshwater mode of life. It is doubtful if they have changed in any major respect since that time so long ago. When we examine a fresh-water jellyfish, we are dealing with an extraordinary biological success story.

It is also ironic that the only traces of the relatives of this unusual animal that long ago shared the ancient sea, and were unable to survive in a changing environment, are frequently found fossilized in rock at the bottom of the pool in which the evolutionary survivors thrive.

Dr. Robert A. Hedeen is a former resident of eastern Maryland, with degrees in zoology and botany. He is a former professor of biological science. He has had 30 scientific papers published, has written numerous magazine articles, and is the author of two books on natural history of the Chesapeake Bay.

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